1. Field of the Invention
The present invention generally relates to semiconductor laser elements and typically to a semiconductor laser element of a high output power and low power consumption for use in an optical disk apparatus, an optical transmission system or the like, and a manufacturing method thereof.
The present invention also relates to an optical disk apparatus and an optical transmission system having such a semiconductor laser element.
2. Description of the Related Art
Among various semiconductor laser elements, a semiconductor laser element of buried ridge structure is able to reduce the parasitic current that does not contribute to laser oscillation and to separate a crystal regrowth interface from the active layer in the element manufacturing stage because of its own structure and is therefore known as having a structure capable of achieving high temperature operation and transverse mode stability while establishing compatibility between high reliability and low power consumption (low threshold current).
The buried ridge structure, of which the portion to be exposed to the atmosphere during the manufacturing process is located apart from the active region at the time of laser oscillation, is therefore able to restrain excess photoabsorption to a low level and secure reliability.
Moreover, providing a current block layer of a small refractive index outside the ridge portion achieves the optical confinement in the horizontal direction by only current constriction and real refractive indices. Since a so-called real refractive index guide structure does not use the photoabsorption for the optical confinement in the horizontal direction, the waveguide loss at the time of laser oscillation is small, and the power consumption can be restrained to a low level.
In fabricating the buried ridge structure semiconductor laser element, it is a general practice to subject a semiconductor substrate having stacked crystal layers including an active layer to two more crystal (re)growth processes.
However, carrying out two crystal regrowth processes is a very large cost increase factor in forming a semiconductor laser element. Accordingly, there is an idea of simplifying the manufacturing process by providing an electrode that forms a Schottky junction with a cladding layer and an ohmic junction with a contact layer (refer to, for example, JP 04-111375 A).
FIG. 34 shows a cross-sectional structure of a semiconductor laser element described in JP 04-111375 A. This semiconductor laser element is manufactured as follows. First, an n-InGaP cladding layer 702, an InGaAs/GaAs strained quantum well active layer 703, a p-InGaP cladding layer 704 and a p-InGaAs contact layer 705 are successively formed on an n-GaAs substrate 701 by the MOCVD (metal-organic chemical vapor deposition) method. Etching is performed until a middle of the p-InGaP cladding layer 704 is reached by the technique of photolithography or the like to form a ridge portion 708 of a forward mesa shape. Thereafter, Ti/Pt/Au and Au—Ge—Ni/Au are deposited as a p-side electrode 706 and an n-side electrode 707, respectively. As a result, a Schottky junction portion is formed between the p-side electrode 706 and the p-InGaP cladding layer 704, while an ohmic junction portion is formed between the p-side electrode 706 and the p-InGaAs contact layer 705. If a current is applied between the p-side electrode 706 and the n-side electrode 707 of the thus fabricated element, then a current flows through only the ohmic junction portion. In this way, current constriction is effected.
With the above-mentioned structure, the crystal growth process, which has been carried out three times in total in an ordinary buried ridge structure, is allowed to be carried out only once, and the manufacturing process steps can consequently be largely reduced.
However, the semiconductor laser element of JP 04-111375 A, in which the current constriction is effected by using the Schottky junction, has been unable to carry out operation with a low threshold current (e.g., 30 mA or less) and a high output power (e.g., output power exceeding 150 mW) as achieved in the conventional semiconductor laser elements of ordinary buried ridge structure. Moreover, it has been difficult to provide an element design matched to the optical characteristic specifications demanded for various applications. Furthermore, the Schottky junction portion has had poor reliability, and long-term reliability has not been able to be achieved.
Reasons for the failure in achieving the operation of a low threshold current and a high power may include the fact that the current constriction property at the Schottky junction portion was so insufficient that the leakage current particularly in a fine stripe-shaped structure could not be sufficiently reduced.
Moreover, a construction capable of establishing compatibility between a low device resistance and the current constriction property is not disclosed. As a result, the device resistance was increased. This has also been a factor that disturbs the high power operation.
Moreover, in order to secure the current constriction property, it is required to use, for the cladding layer, materials such as InGaP having a great energy discontinuity (barrier) ΔEv in the valence band (so as to be lattice matched to GaAs) and AlGaAs having a high Al mole fraction, and there has been little scope for changing the refractive index of the cladding layer. Therefore, the degree of freedom in designing the optical characteristics has been limited. Furthermore, no Schottky junction structure having resistance to breakdown has been disclosed, and so far there have been only structures that lack long-term reliability.
In addition, the semiconductor laser element and the manufacturing method disclosed in JP 04-111375 A has had a problem that the optical characteristics of the laser element are largely influenced by the variation in the layer thickness of the remaining p-InGaP cladding layer after etching, occasionally causing variation in the far-field pattern (FFP) in the horizontal direction and a deterioration in the stability of the transverse mode. Furthermore, due to the variation in the InGaP layer thickness, the characteristic of the Schottky junction formed on that layer may also vary, and this may lead to an insufficient current constriction. Moreover, since the refractive index of the InGaP layer that is a lattice matching condition is uniquely determined, adjustment thereof is only achievable by changing the InGaP film thickness when making an optical design. Thus, the degree of freedom of design is small. There is a further problem that use of InGaP for the p-cladding layer tends to increase ΔEv, which may result in a limit to the injection efficiency of holes into the active layer.